The present invention relates to a gas separation assembly and membrane gas separation processes wherein the assembly is comprised of hollow fiber membranes capable of selectively permeating one component of fluid mixture over other components. More particularly, the invention relates to a membrane gas separation assembly which provides an internal countercurrent sweep and processes that utilize this assembly.
It is known in the art to use various hollow fiber membrane gas separation devices for separating gas mixtures. Normally, these separation devices are designed so that the gas mixture can be brought into contact with the hollow fiber membrane therein under a partial pressure differential one or more highly permeable components of the fluid mixture are being separated from the less permeable components. The hollow fiber membrane allows the more readily permeable component of the fluid mixture to permeate into the permeate side of the hollow fiber membrane while retaining a substantial portion of the less readily permeable component of the fluid mixture on the non-permeate side of the hollow fiber membrane. The permeated and non-permeated components are removed through or recovered from at least one permeate outlet and at least one non-permeate outlet, respectively.
In some instances the membrane gas separation devices, (assemblies) are designed to provide a purge or a sweep gas on the permeate side of the membrane. The use of a sweep gas on the permeate side of the membrane is beneficial in certain gas separation processes, such as gas dehydration processes, since it decreases the permeate side partial pressure of the more highly permeable component thus allowing the gas mixture to be more thoroughly stripped of the more readily permeable component. The sweep gas is typically flown counter currently to the direction of the feedxe2x80x94non-permeate flow. The use of a dry sweep gas can improve the product gas dryness as well as the productivity of the membrane device. A portion of the dry product gas is frequently utilized as the sweep gas generating an internal reflux system.
The gas separation assembly that provides for sweep or purge gas introduction generally comprises an annular hollow fiber membrane bundle in an enclosure or a shell having a fluid feed inlet, a non-permeate outlet, a permeate outlet and a sweep or purge gas inlet. Examples of such membrane assemblies can be found in U.S. Pat. Nos. 3,499,062, 4,718,921, 5,108,464 and 5,026,479. These fluid separation devices, however, require external plumbing and valves to regulate the flow of the sweep gas to be fed to the sweep gas inlet port. In some gas separation applications, such as gas drying, a portion of the non-permeate product (the dry gas) is used as the sweep gas. The need to manifold the dry sweep gas external to the gas separation apparatus adds to the size and the complexity of the device.
Several attempts have been made to provide an internal sweep gas arrangement and an internal sweep gas flow control. U.S. Pat. Nos. 5,411,662 and 5,525,143 disclose such integral hollow fiber devices.
The hollow fiber membrane assemblies with integral internal purge arrangements, however, can have a number of disadvantages. The purge flow does not shut down automatically when the product (non-permeate) gas is not being withdrawn from the device. The feed flow to the assembly must be shut down or a valve on the purge flow line must be installed and closed to prevent a continuous loss of the feed gas through the purge conduit. Furthermore, the purge flow will remain constant irrespective of product draw or the required product dew point. Several attempts have been made to regulate the purge flow rate according to the feed or product flow rates or the level of product dryness required. Examples can be found in U.S. Pat. Nos. 5,160,514, 6,006,383 and the U.S. Pat. No. 5,411,662 referenced above and in JP09057043. However, these designs are complicated and difficult to implement. Thus, there still remains a need in the field for an improved hollow fiber gas separation assembly with internal reflux system.
Accordingly, it is an object of the invention to provide means by which the operation of the gas separation apparatus equipped with a reflux system can be carried out without external plumbing and valves. It is another object of the invention to provide means by which the gas separation apparatus having a purging means can be easily implemented and operated. It is a further object of this invention to provide a means to reduce gas losses through the purge gas conduit when the membrane separation assembly is not in operation. It is a further object of the present invention to provide a means to adjust the volume of the purge flow according to the amount of non-permeate gas withdrawn without the need for external intervention, outside energy sources or complicated peripheral devices.
The present invention provides a hallow fiber membrane gas separation assembly having a counter current sweep on the permeate side of the hollow fibers with a portion of the product gas wherein the sweep gas is introduced internally to the assembly. The assembly is comprised of an elongated casing or shell having a feed gas inlet and permeate and product gas outlets. The outlets are positioned essentially at the same end of the casing, and the feed inlet is appropriately located between the tubesheets. The casing encloses a multiplicity of hollow fiber membranes positioned around an inner, tubular core member. The hollow fibers extend between two tubesheets, each end of hollow fibers terminating in a tubesheet and being opened to allow unobstructed gas flow into and out of the hollow fiber bores. Means such as O-rings to secure and seal tubesheets to the casing in fluid tight relationship are further provided. The ends of the tubular core member are open through the ends of the tubesheets. The assembly is provided with at least one purge flow control orifice positioned in the tubular core member that directs predetermined amount of the product gas into the bores as a counter current sweep. According to one embodiment of the present invention an on-off valve is positioned in the tubular core member that substantially shuts off the flow of the purge gas when the product gas is not withdrawn from the assembly.
According to another embodiment of the present invention a purge flow control valve is positioned in the tubular core member that regulates the volume of the purge gas in proportion to the amount of product gas withdrawn from the assembly.
The invention further provides for gas dehydration processes that utilize the disclosed novel gas separation apparatuses. The gas dehydration processes of this invention are designed to remove predetermined amounts of the water vapor contained in the feed gas wherein the amount of sweep gas utilized to purge the permeate side of hollow fiber membranes is minimized. The sweep gas flow is generally from about 1% to about 80%, preferably from 5% to about 60%, of the net flow rate of the dehydrated product gas.